Abstract
A systematic and detailed analysis of adsorption and diffusion properties of xylene isomers over Beta zeolites with different mesoporosity was conducted. Adsorption isotherms of xylene isomers over microporous and mesoporous Beta zeolites through gravimetric methods were applied to investigate the impact of mesopores inside Beta zeolites on the adsorption properties of xylene isomers in the pressure range of lower 20 mbar. It is seen that the adsorption isotherms of three xylene isomers over microporous and mesoporous Beta zeolites could be successfully described by the single-site Toth model and the dual-site Toth model, respectively. The enhanced adsorption capacities and decreased Henry’s constants (KH) and the initial heats of adsorption (Qst) for the all xylene isomers are observed after the introduction of mesopores in the zeolites. For three xylene isomers, the order of Henry’s constant is o-xylene > m-xylene > p-xylene, whereas the adsorption capacities of Beta zeolite samples for xylene isomers execute the following order of o-xylene > p-xylene > m-xylene, due to the comprehensive effects from the molecular configuration and electrostatic interaction. At the same time, the diffusion properties of xylene isomers in the mesoporous Beta zeolites were also studied through the desorption curves measured by the zero length column (ZLC) method at 333–373 K. It turned out that the effective diffusion time constant (Deff/R2) is a growing trend with the increasing mesoporosity, whereas the tendency of the activation energy is just the reverse, indicating the contribution of mesopores to facilitate molecule diffusion by shortening diffusion paths and reducing diffusion resistances. Moreover, the diffusivities of three xylene isomers in all Beta zeolites follow an order of p-xylene > m-xylene > o-xylene as opposed to KH, conforming the significant effects of adsorbate-adsorbent interaction on the diffusion.
Highlights
Zeolites are typical microporous crystalline materials with narrow pore size distribution ranging from0.3 to 1.5 nm, having a widespread application in adsorption, separation, and catalysis with its unique properties, such as developed ordered porous structure, high specific surface area, and strong acidity, high hydrothermal and thermal stability [1,2,3]
It is observed that the isotherms of three xylene isomers on the Beta-0 display a sharp increase of adsorption capacities at low pressure region corresponding to micropore filling and turn out to be a plateau at relative high pressures, which represent the typical characteristics of type-I adsorption isotherm
The fact that the system is controlled by kinetics was verified by measuring zero length column (ZLC) desorption curves of xylene isomers at two different flow rates under the same conditions
Summary
0.3 to 1.5 nm, having a widespread application in adsorption, separation, and catalysis with its unique properties, such as developed ordered porous structure, high specific surface area, and strong acidity, high hydrothermal and thermal stability [1,2,3]. Among which, introducing mesopores into the original structure of microporous zeolites is one of the most efficient methods [5,6,7] Such mesostructured zeolite materials have the advantages of micropores, which ensure acidity and shape selectivity, and the advantages of mesopores, which promote the adsorption of molecules [8,9,10] and diffusion rate for mass transfer within zeolites [8,11,12,13]. In consideration of the essentiality for studying the adsorption and mass transport of molecules in the pores of nanoporous materials, where sorption and reaction sites are located, to understand catalytic and separation processes of organic molecules, in the present study, three xylene isomers were employed as probe molecules to explore the adsorption and diffusion behaviors of xylene isomers molecules on the mesoporous Beta zeolites by their adsorption isotherms and ZLC desorption curves. The impacts of mesopore in Beta zeolites on the adsorption and diffusion of xylene isomers with different molecular configuration will be investigated
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